Final Set A Answer Scheme

PART A (Answer all question)

Q1 With reference to Electrical Power Distribution onboard merchant ship,

a. With the aid of sketch, explain what is an insulated neutral system and why it is

preferred.

Answer

Insulated system totally electrically insulated from earth ( Ship’s Hull) An insulated neutral system is one that has its neutral electricity

isolated from the earth.

The priority requirement onboard a ship is to maintain continuity of the

electrical supply to an essential equipment in the event of single earth-

fault occurring.

(6M)

b. Briefly explain how the emergency generator can be started automatically in event of

black out.

Answer Starting automatically – initiated by relay which monitors normal main

supply.

Falling mains frequency/voltage cause ‘start-up’ relay to operate

generator starting equipment

Arrangement for starting – electrical, pneumatic, hydraulic

(3M)

c. Draw a layout typical power distribution system with clearly labels all principal

components.

Answer:

(7M)

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d. Describe the effect to the ship electrical system rated at 440V if she is supplied with

380V during her emergency dry docking.

Answer Lower voltage may cause motors to run slower and overheat. May cause

motor to stall. Amount of power that motor draw are proportion to voltage and current (P=IV) If V is low, I will increase to produce same amount of power. Eg; if V is less

10%, current will increase 10%. If amount of I exceed the motor nameplate rating, motor will be stress and

overheat. More heat and longer exposure to it will damage or shorten the lifespan of the motor.

(4M)

Q2

With reference to the electrical shock and safety,

a. Describe an electric shock and the conditions that affect the electrical shock

Answer

As we knew, electric shock is due to flow of current through the body. This is often from hand to hand or hand to foot. A shock current of 15mA AC or DC may be fatal.

The size of electric shock current is related to the following factors:1. Applied voltage 2. Body resistance.

The size of human body resistance also depends upon other factors as state of health, degree of contact with live wires, perspiration or dampness of skin.

(5M)

b. A.C compressor motor no 2 was due for overhauling. List down in order the 7 step

safety procedures required before doing any electrical maintenance job onboard

(7M)

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ship.

Answer

c. List down the procedure of saving a person who was suspected “Froze on the circuit”Answer

(8M)

Q3

With reference to cell battery and battery charger onboard merchant ship,

a. Explain why the batteries are considered as important in the electrical system.

Answer Batteries are used as backup power supply besides emergency

generator. Batteries are used for some emergency lighting and navigation

equipment because battery is the only power source in an emergency.

(4M)

b. Explain why the lead acid batteries are more prefer compared to alkaline battery. (6M)

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Answer ( Any 6 of this below – 1 marks each) Initial cost of alkaline battery Electrolyte for alkaline – corrosive Battery container – steel material for alkaline but for lead acid only plastic. Cell voltage – 1.2V for alkaline compared to 2V for lead acid battery Number of cell required for alkaline more than lead acid because of cell

voltage Space requirement – because 24V supply need 20 cells of alkaline battery

but for lead acid only need 12 cells. First aid facility – boracic acid need for alkaline but lead acid only need water. Performance indicator – need for monitor condition of alkaline battery

c. With aid from sketch, briefly explain the operation of battery charger/UPS.

Answer(10M)

Typical continuous UPS DC supported supply system

Essential DC services supplied from 440V through charger 1 - continuously in

trickle charges

During power loss, battery 1 maintains transitional supply while emergency

generator restores power to emergency board & charger 2

Either battery is available for few hours if both generators are unavailable

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PART B: (Answer two (2) question only )

Q4With reference to the general requirement and lighting system on board of merchant ship :

a. Describe why shore equipment’s are not suitable for marine use.

Answer ( any 6 of below list – 1 mark each)1. Marine environment2. High surrounding vibration3. Voltage fluctuation4. Climatic conditions 5. Temperature 6. Continuous operation – the best is if it can last until ship lifetime.7. Reliability

(6M)

b. Explain on the following :

i. Incandescent lamp

Answer Most common used for general lighting with simple filament type

Operation by current passed through fine wire tungsten filament, raises

temperature up to 3000°C, thus becomes incandescent (it glows)

Glass bulb is filled with inert gas (nitrogen, argon etc) for reduce filament

evaporation

3 type of filament – ordinary filament, Rough service, Special coiled-coil

filament

Other variations – clear glass bulb, inside frosted glass bulb (pearl) to

reduce glare, tubular construction, internal reflector lamps, decorative

lamps (e.g. candle shape) and heating lamps.

ii. Fluorescent lamp

Answer Light output is generated by current flow in electric arc between two

electrodes through gas and metal vapour inside seal glass bulb or tube.

Common metal vapour used – mercury & sodium Suitable voltage applied between electrodes causes an arc discharge

through the gas.

(5M)

(5M)

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Gas ionization will create visible light directly or by secondary emission

from phosphor coating inside lamp glass.

Current must control to maintain desired light output – ballast (iron cored

inductor (choke), special transformer or electronic ballast circuits.

c. Explain how the variation on voltage can effect the performance of the

lighting.

Answer All lamps are designed to produce rated luminous output at rated voltage.

Overvoltage on incandescent lamp – brighter and whiter light, increase

filament temperature, reduced operating life.

5% increase in voltage will reduce lamp life by 50%.

Reduction in supply voltage for GLS will increase operating life but duller

and reddish light.

(4M)

Q5With reference to AC induction motor that fitted onboard ship:

a. State the relationship between frequency, number of poles and the speed of the motor.

AnswerWhen the three phase supply is connected to the stator winding the currents

which flow in the windings produce a multi-pole magnetic field. This field

rotates at a speed called synchronous speed. The value of synchronous

speed depends on how many magnetic poles the stator winding produces and

on the frequency of the supply connected to the stator winding. Thus, we can

conclude the relationship between frequency, speed and number f poles as

follows:

N=f/p

Where,

n = synchronous speed (rev/speed)

f = supply frequency (Hz)

p = number of poles (in pairs)

(6M)

b. Explain the definition of Single Phasing in three phase motor. State a method to

(6M)

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protect motor from single phasing situation.

AnswerSingle-phasing is usually caused when one of the three back-up fuses blows or if

one of the contactor contacts is open-circuited.

The effect of single-phasing is to increase the current in the two remaining lines

and cause the motor to become very noisy due to the uneven torque produced in

the rotor.

An increase in line current due to single-phasing will be detected by the protective

OCR. The three thermal elements of an OCR are arranged in such a way that

unequal heating of the bi-metal strips causes a differential movement which

operates the OCR switch contacts to trip out the motor contactor.

c.

Briefly explain how would you going to repair & refurbish the damaged induction

motor that having earth fault (low insulation) which is used as a hoisting motor for

midship crane.

( Your answer should comprise the necessary precaution and safety awareness

as well)

Answer1. Prepare JSA, RA and work permit

2. Switch off breaker, pull fuse and put Lock Out / Tag out

3. Test the circuit dead by voltmeter, take insulation reading for motor.

4. Disassembled the rotor and stator carefully and ensure the rotor not

rubbing the stator

5. Clean the stator winding with clean cloth to eliminate dirt and moisture. If

stator was oily/greasy, clean with degrease ( Electro Solvent). If any

scratch on stator, re-varnish with epoxy.

6. Dry the stator winding either by current injection, heating by using heating

lamp or oven.

7. Take again insulation reading if any improvement, the insulation reading

was good if 20Mohm and above.

8. Re-grease or change ball bearing if necessary, box up back and test run.

(8M)

Q6With reference to DC motor and DC Generator:

a. Basic construction for DC motor consists of an armature winding and field

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winding. With the aid of sketch, explain how you can vary the speed for a DC

motor.

AnswerHave 3 ways can vary the speed for a DC motor.

1. Vary the voltage supplied to the armature circuit while holding the

field constant.

2. Insert resistance in series with the armature circuit.

3. Vary the field current while holding the armature supply voltage

constant.

(10M)

b. State the difference of shunt wound DC motor compared to the series wound DC

motor

Answer

(4M)

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When the field winding is connected in parallel with the armature, the

machine is said to be shunt wound

If the field winding is connected in series with the armature, the machine is said to be series wound.

c. Draw line diagram of a compound wound DC generator and show its voltage-load

characteristics. What the meant by over, under and level compounding of these

generators.

Answer

Having mixture of shunt & series windings – to combine the

advantages.

Different Connection – different voltage regulation characteristics on

load.

(6M)

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long shunt compound generator

Short shunt compound generator

END OF QUESTION

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